Maths question regarding curved bay window
@batFINGER's or something like it, is clearly the most accurate way, but if eyeballing is enough, you can use the Spin tool to create an arc between 3 points.
If the 2 end vertices are not aligned to a convenient set of axes, you may have to make a Custom Orientation from them.
Select the 2 end vertices, and ShiftS Cursor to selected.
ShiftD duplicate one of the end vertices, and call up the Spin tool. Adjust the spin center in the perpendicular to the two ends to pass the arc of the spun vertex through the third point, and its angle to span the end points.